The present invention relates to a prepolymerization catalyst for use in a gas phase polymerization of olefins. More particularly, the invention relates to a prepolymerization catalyst for use in a gas phase polymerization of olefins showing a high activity in gas phase polymerization, not causing a formation of aggregates and coarse polymer particles markedly during the prepolymerization, having high bulk density and excellent fluidity, not causing an entraining of the prepolymerization catalyst and a product powder out of a fluidized bed markedly and nearly completely free from a formation of polymer aggregates at the time of the gas phase polymerization, and capable of giving an olefin polymer low in the content of cold xylene-soluble fraction, and a process for producing the same.
Since olefin polymers have high mechanical property such as strength, good appearance such as transparency, and excellent moldability or handling property such as film-forming property, olefin polymers are extensively used as a material for films and molded articles. Among the olefin polymers, polyethylenes such as ethylene homopolymers and linear low-density polyethylenes (LLDPE), which are ethylene-xcex1-olefin copolymers, are especially suitable for use as film-forming materials.
High-activity catalysts for production of olefin polymers have a very high industrial value, because they can be used in the gas phase olefin polymerization process in which the de-ashing step is simplified. However, when an olefin is polymerized by a gas phase polymerization process using a high-activity catalyst, the polymerization is accompanied by generation of a large quantity of heat, and thereby fusion and aggregation of the resulting olefin polymer may take place and it may become difficult to continue the polymerization of olefin or produce olefin polymer.
As a method for preventing the above-mentioned aggregation of polymer generated in olefin polymerization, there is known a method of using, as a catalyst for gas phase polymerization of olefin, a prepolymerization catalyst obtained by prepolymerizing ethylene and/or an xcex1-olefin on an olefin-polymerizing catalyst.
For instance, JP-A-59-30806, JP-A-7-196720 and JP-A-8-337611 disclose a prepolymerization catalyst which is a powder of an xcex1-olefin prepolymer having such a particle dimension distribution that the mass-average diameter (Dm) is 80-300 xcexcm and the ratio of number-average diameter (Dn) to the mass-average diameter (Dm) is smaller than or equal to 3, and also disclose that olefin polymers such as ethylene homopolymer or ethylene-butene-1 copolymer can be obtained by continuously effecting an olefin polymerization by using the above-mentioned powdery prepolymer in a gas phase polymerization.
However, if a catalyst having higher activity such as those described in JP-A-11-322833 is subjected to a prepolymerization in a suspension polymerization system, in some cases the prepolymerization catalyst is not smoothly drawn out of the suspension polymerization reactor because coarse granules or aggregates of polymer are formed and fluidity of the prepolymerization catalyst is extremely insufficient due to its low bulk density. Furthermore, there was a problem that when fluidity of the prepolymerization catalyst is insufficient, the quantity of prepolymerization catalyst fed cannot be kept constant at the time of the feeding of the prepolymerization catalyst to a gas phase fluidized bed type gas phase polymerization reactor together with a gas stream, which causes fluctuation of olefin polymerization temperature or formation of olefin polymer aggregates in the gas phase fluidized bed type gas phase polymerization reactor and thereby makes it difficult to polymerize the olefin steadily.
Under the above-mentioned circumstances, it is desired to develop a prepolymerization catalyst for use in the gas phase polymerization of olefins high in the activity in gas phase polymerization, not causing formation of aggregates and coarse granules markedly at the time of prepolymerization, high in bulk density, excellent in fluidity, causing no remarkable entraining of the prepolymerization catalyst and the resulting powdery olefin polymer out of the fluidized bed at the time of gas phase polymerization, nearly completely free from formation of polymer aggregates, and giving an olefin polymer low in the content of cold xylene-soluble fraction.
It is an object of the present invention to provide a prepolymerization catalyst for use in the gas phase polymerization of olefins high in the activity in gas phase polymerization, not causing formation of aggregates and coarse granules markedly at the time of prepolymerization, high in bulk density, excellent in fluidity, causing no remarkable entraining of the prepolymerization catalyst and the resulting powdery olefin polymer out of the fluidized bed at the time of gas phase polymerization, nearly completely free from formation of polymer aggregates, and giving an olefin polymer low in the content of cold xylene-soluble fraction, and a process for producing the same.
In view of the above, the present inventors have conducted extensive studies to find that the problem mentioned above can be solved by a prepolymerization catalyst comprising a solid catalyst component having a specified weight-average particle diameter, an organoaluminum compound and a prepolymer of ethylene and/or at least one xcex1-olefin and having a specified aluminum-titanium ratio, a specified weight ratio of prepolymerization catalyst/solid catalyst component, a specified volatile material content and a specified intrinsic viscosity. Based on this finding, the present invention has been accomplished.
Thus, the present invention relates to a prepolymerization catalyst for use in a gas phase polymerization of olefins which comprises (A) a solid catalyst component comprising magnesium, halogen, titanium and an electron donor and having a weight-average particle diameter of 15-45 xcexcm, (B) at least one organoaluminum compound and (C) a prepolymer of an ethylene and/or at least one xcex1-olefin, wherein the molar ratio of aluminum to titanium (Al/Ti ratio) contained in said prepolymerization catalyst is 3 to 11 (mol/mol), the weight ratio of the prepolymerization catalyst to the solid catalyst component (prepolymerization catalyst/solid catalyst component ratio) is 2 to 35 (g/g), the content of volatile materials (VM) in the prepolymerization catalyst is 2.0% by weight or less, and the intrinsic viscosity [xcex7] of the prepolymerization catalyst measured in tetralin at 135xc2x0 C. is 2.0 dl/g or less, and a process for a production of the prepolymerization catalyst.
The present invention further relates to a process for producing an olefin polymer which comprises polymerizing olefins by means of a gas phase fluidized bed using the above-mentioned prepolymerization catalyst for gas phase polymerization.
Next, details of the present invention will be described below.
As used in the present invention, the term xe2x80x9cpolymerizationxe2x80x9d means not only a homopolymerization, but also inclusively means a copolymerization; and the term xe2x80x9cpolymerxe2x80x9d means not only a homopolymer but also means a copolymer inclusively.
As used in the present invention, the term xe2x80x9colefinxe2x80x9d means an olefin having 2 or more carbon atoms which include, for instance, ethylene, propylene, butene-1, pentene-1, hexene-1, heptene-1, octene-1, decene-1,3-methylpentene-1,4-methylpentene-1, and the like, and it means preferably ethylene, propylene, butene-1, hexene-1, octene-1 and 4-methylpentene-1, and further preferably ethylene, propylene, butene-1 and hexene-1.
The term xe2x80x9cgas phase polymerizationxe2x80x9d used in the present invention means a polymerization process used for polymerizing olefins in a gas phase. It is known that a gas phase polymerization process requires only a smaller investment and a lower energy cost as compared with other polymer-producing processes such as a suspension polymerization process and a solution polymerization process. In the gas phase polymerization, a gas phase fluidized bed type reactor is usually used in order to make a smooth progress of the polymerization reaction. Herein, the term xe2x80x9cgas phase fluidized bed type reactorxe2x80x9d means a reactor utilizing a fluidized bed, in which a reaction is conducted in a bed of powdery particles filled in the reactor and kept in a floating state by the action of a gas introduced through a plate with many pores which is provided at the bottom of the apparatus (this plate is called a gas-dispersing plate) (this state of the floating particles is called a xe2x80x9cfluidized bedxe2x80x9d).
As used in the present invention, the term xe2x80x9cprepolymerizationxe2x80x9d means a process of polymerizing a small quantity of olefins by the use of (A) a solid catalyst component comprising magnesium, halogen, titanium and an electron donor and having a weight-average particle diameter of 15-45 xcexcm and (B) at least one organoaluminum compound, to form an olefin polymer on the solid catalyst component; and a product obtained by the prepolymerization is a prepolymerization catalyst.
As the olefins which can be used in the prepolymerization of the present invention, the same olefins as used in the above-mentioned gas phase polymerization process can be referred to. The olefins may be one species of olefin or a combination of two or more olefins.
A process for producing the prepolymerization catalyst is not particularly limited, and a suspension polymerization process, a gas phase polymerization process and the like can be referred to. Preferably, it is a suspension polymerization process. It may be produced by the use of any of batch method, semi-batch method and continuous method.
In a case where the prepolymerization catalyst is produced by a suspension polymerization process, a solvent may be a hydrocarbon having 20 or less carbon atoms. Examples thereof include saturated aliphatic hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, hexane, heptane, octane, decane and the like, and aromatic hydrocarbons such as toluene, xylene and the like. Among these hydrocarbons, n-butane, hexane, heptane and toluene are preferable, and n-butane and hexane are further preferable.
A slurry concentration in the suspension polymerization may be a usual concentration. A preferable slurry concentration is 0.001-0.5 g/ml and a further preferable concentration is 0.005-0.4 g/ml, as expressed in terms of solid catalyst quantity per milliliter solvent.
Although the speed of stirring in the polymerization reactor for prepolymerization is not particularly limited, it is preferably a stirring speed at which the solid catalyst component and the prepolymerization catalyst can be kept in a floating state, namely a speed not lower than the critical speed necessary for floatation of stirred particles.
The polymerization temperature of the prepolymerization may be a usual polymerization temperature. It is preferably a temperature ranging from xe2x88x9210xc2x0 C. to 100xc2x0 C., and further preferably from 0xc2x0 C. to 70xc2x0 C. The polymerization pressure of the prepolymerization may be a usual polymerization pressure. It is preferably a pressure ranging from atmospheric pressure to 4.0 MPaG.
As the method for controlling the intrinsic viscosity [xcex7] of the prepolymerization catalyst measured in tetralin at 135xc2x0 C., a method using a chain transfer agent such as hydrogen, an organometallic compound and the like, and a method of controlling the prepolymerization temperature, etc. can be referred to. A preferable method is the method using hydrogen. As the method using hydrogen, a method of adding hydrogen before feeding ethylene, a method of adding hydrogen with controlling a flow rate simultaneously with feeding ethylene, etc. can be referred to.
The prepolymerization catalyst is generally obtained in a state of dryness. As the method for drying, a method of drying a prepolymerization catalyst while flowing a nitrogen gas stream, a method of vacuum drying using a vacuum pump, etc. can be referred to.
As used herein, the term xe2x80x9cmagnesiumxe2x80x9d which is contained in the solid catalyst component (A) means a magnesium atom belonging to Group 2 of the periodic table, and the term xe2x80x9ctitaniumxe2x80x9d means a titanium atom belonging to Group 4 of the-periodic table.
The term xe2x80x9chalogenxe2x80x9d which is contained in the solid catalyst component (A) means a halogen which is an element belonging to Group 17 of the periodic table. Examples thereof include a chlorine atom, a bromine atom, an iodine atom and the like, and it is preferably a chlorine atom.
The term xe2x80x9celectron donorxe2x80x9d which is contained in the solid catalyst component (A) means an organic compound containing at least one member of an oxygen atom, a sulfur atom, a nitrogen atom and/or a phosphorus atom. Examples thereof include amines, sulfoxides, ethers and esters. It is preferably an ether or an ester.
As the ethers, dialkyl ethers can be referred to. Examples thereof include diethyl ether, dibutyl ether, tetrahydrofuran and the like. It is preferably dibutyl ether or tetrahydrofuran.
As the esters, esters of saturated aliphatic carboxylic acids, esters of unsaturated aliphatic carboxylic acids, esters of alicyclic carboxylic acids, esters of aromatic carboxylic acids, and the like can be referred to. Examples thereof include ethyl acetate, ethyl acrylate, ethyl methacrylate, butyl benzoate, dibutyl succinate, dibutyl malonate, dibutyl maleate, dibutyl itaconate, di-n-butyl phthalate, diisobutyl phthalate, di-2-ethylhexyl phthalate, di-n-octyl phthalate, and the like. Among these esters, preferred are di-2-ethylhexyl phthalate and di-n-octyl phthalate.
The solid catalyst component (A) is preferably a product obtained by bringing a solid catalyst component precursor containing magnesium, titanium and a hydrocarbyloxy group into contact with a halogen compound of an element belonging to Group 14 of the periodic table and an electron donor to obtain a contacted product, followed by further bringing the contacted product into contact with a compound having a Ti-halogen bond.
Said solid catalyst component precursor containing magnesium, titanium and a hydrocarbyloxy group is preferably a solid product containing a trivalent titanium atom which can be obtained by reducing a titanium compound represented by the following general formula:
Ti(OR1)aX4xe2x88x92a
wherein R1 is a hydrocarbon group having 1-20 carbon atoms, X is a halogen atom and a is a number satisfying 0 less than a less than 4, with an organomagnesium compound in the presence of an organosilicon compound having a Sixe2x80x94O bond.
As said organosilicon compound having a Sixe2x80x94O bond, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane and the like can be referred to, among which preferred is tetrabutoxysilane.
As the hydrocarbon group (R1) of the titanium compound represented by the general formula Ti(OR1)aX4xe2x88x92a, wherein R1 is a hydrocarbon group having 1-20 carbon atoms, X is a halogen atom and a is a number satisfying 0 less than a less than 4, for example, a methyl group, an ethyl group, a propyl group, a butyl group and the like can be referred to, among which preferred is a butyl group.
As the halogen atom (X) of the titanium compound represented by the general formula Ti(OR1)aX4xe2x88x92a, a chlorine atom, a bromine atom, an iodine atom and the like can be referred to, among which preferred is a chlorine atom. The number a is 1, 2, 3 or 4, and it is preferably 4.
As the titanium compound represented by the general formula Ti(OR1)aX4xe2x88x92a, for example, butoxytrichlorotitanium, dibutoxydichlorotitanium, tributoxychlorotitanium, tetrabutoxytitanium and the like can be referred to, among which preferred is tetrabutoxytitanium.
As the organomagnesium compound, Grignard compounds having a Mg-carbon bond and the like can be referred to. Examples thereof include methylchloromagnesium, ethylchloromagnesium, propylchloromagnesium, butylchloromagnesium and the like, among which preferred is butylchloromagnesium.
As the halogen compound of an element belonging to Group 14 of the periodic table to be contacted with the solid catalyst component precursor, halogen compounds of carbon atom or silicon atom can be referred to. It is preferably a halogen compound of silicon atom represented by the following general formula:
SiR24xe2x88x92bXb
wherein R2 is a hydrocarbon group having 1-20 carbon atoms, X is a halogen atom and b is a number satisfying 0 less than b less than 4.
As the hydrocarbon group (R2) of the silicon compound represented by the general formula SiR24xe2x88x92bXb, wherein R2 is a hydrocarbon group having 1-20 carbon atoms, X is a halogen atom and b is a number satisfying o less than b less than 4, for example, a methyl group, an ethyl group, a propyl group, a butyl group and the like can be referred to, among which preferred is a butyl group.
As the halogen atom (X) of the silicon compound represented by the general formula SiR24xe2x88x92bXb, a chlorine atom, a bromine atom, an iodine atom and the like can be referred to, among which preferred is a chlorine atom. The number b is 1, 2, 3 or 4, and it is preferably 4.
As the silicon compound represented by the general formula SiR24xe2x88x92bXb, for example, butyltrichlorosilane, dibutyldichlorosilane, trichlorobutylsilane, tetrachlorosilane and the like can be referred to, among which preferred is tetrachlorosilane.
As the electron donor to be brought into contact with the solid catalyst component precursor, the same ones as mentioned above can be referred to.
As the halogen in the compound having a Tixe2x80x94O bond to be further brought into contact with the contacted product, which is obtained by contacting the solid catalyst component precursor with the halogen compound of an element belonging to Group 14 of the periodic table and the electron donor, a chlorine atom, a bromine atom, an iodine atom and the like can be referred to, among which preferred is a chlorine atom.
As the compound having a Ti-halogen bond, for example, tetrachlorotitanium, trichlorobutoxytitanium, dichlorodibutoxytitanium, chlorotributoxytitanium and the like can be referred to, among which preferred is tetrachlorotitanium.
Weight-average particle diameter of the solid catalyst component (A) of the present invention is 15-45 xcexcm, and preferably 20-35 xcexcm.
When the weight-average particle diameter of the solid catalyst component (A) is smaller than 15 xcexcm, it can occur that the prepolymerization catalyst is entrained out of the gas phase fluidized bed to obstruct an effective use of the catalyst or that the prepolymerization catalyst adheres to the wall of enlarged part of the gas phase fluidized bed type reactor to generate a formation of aggregates. When the weight-average particle diameter of the solid catalyst component (A) exceeds 45 xcexcm, it can occur that bulk density of the resulting polymer powder decreases or the content of cold xylene-soluble fraction thereof increases.
As used in the present invention, the term xe2x80x9corganoaluminum compound (B)xe2x80x9d means a compound having at least one Al-carbon bond. Examples thereof include trimethylaluminum, triethylaluminum, triisobutylaluminum and the like, among which preferred is triethylaluminum.
As used in the present invention, the term xe2x80x9cprepolymer (C) of ethylene and/or at least one xcex1-olefinxe2x80x9d means a polymer of ethylene and/or at least one xcex1-olefin which is formed on the solid catalyst component (A) by contacting the solid catalyst component (A), an organoaluminum (B) and olefins which are ethylene and/or at least one xcex1-olefin.
As the xcex1-olefin, olefins having 3 or more carbon atoms among the olefins used in the above-mentioned gas phase polymerization can be referred to. Examples thereof include propylene, butene-1, hexene-1, pentene-1,3-methylpentene-1, and the like.
In the prepolymerization catalyst of the present invention, the ratio of aluminum atom to titanium atom [Al/Ti ratio (mol/mol)] contained therein is 3 to 11, and preferably 4 to 8. When the Al/Ti ratio is less than 3, it can occur that a polymerization rate in the prepolymerization lowers or an activity of catalyst in gas phase polymerization lowers. When the Al/Ti ratio exceeds 11, it can occur that the olefin polymer obtained by gas phase polymerization contains an increased quantity of cold xylene-soluble fraction.
In the prepolymerization catalyst of the present invention, the ratio of the prepolymerization catalyst to the solid catalyst component (prepolymerization catalyst/solid catalyst component) is 2 to 35 (g/g), and preferably 4 to 25 (g/g). When the (prepolymerization catalyst/solid catalyst component) ratio is smaller than 2 g/g, it can occur that the gas phase polymerization generates much quantity of heat due to the polymerization and thereby the olefin polymer formed by the reaction melts or aggregates to make the progress of gas phase polymerization difficult. When the (prepolymerization catalyst/solid catalyst component) ratio exceeds 35 g/g, it can occur that the equipment for prepolymerization must be designed so as to have an extremely large dimension or the powder obtained by the gas phase polymerization has a lowered bulk density.
Although the weight-average particle diameter of the prepolymerization catalyst of the present invention is not particularly limited, it is preferably 15 to 75 xcexcm and more preferably 20 to 75 xcexcm, in the point of the bulk density of the powder obtained in the gas phase polymerization and the entraining of the prepolymerization catalyst and the powder formed in the gas phase polymerization.
In the prepolymerization catalyst of the present invention, the term xe2x80x9ccontent of volatile materials (VM)xe2x80x9d means the quantity of the solvent used for the prepolymerization and unreacted olefins remaining in the prepolymerization catalyst. Content of the volatile materials (VM) is 2.0% by weight or less, preferably 1.0% by weight or less, and most preferably 0% by weight. When the content of volatile materials (VM) exceeds 2.0% by weight, a fluidity of the prepolymerization catalyst becomes low.
Intrinsic viscosity [xcex7] of the prepolymerization catalyst of the present invention measured in tetralin at 135xc2x0 C. is 2.0 dl/g or less, preferably 1.7 dl/g or less, and further preferably 1.5 dl/g or less. When the intrinsic viscosity [xcex7] exceeds 2.0 dl/g, bulk density of the prepolymerization catalyst can become low or many coarse particles can be formed, due to which a fluidity of the prepolymerization catalyst becomes low. The decrease in fluidity makes a cause of defective drawing-out of prepolymerization catalyst from the reactor or an insufficient feeding of prepolymerization catalyst into the reactor of the subsequent step, namely the step of gas phase polymerization constituting a main step of the process.
The use of the prepolymerization catalyst of the present invention makes it possible to effect a gas phase polymerization, constituting a main step of the polymerization process, effectively. The gas phase polymerization can be carried out according to known procedure, by using the above-mentioned fluidized bed reactor. Generally, the reaction temperature is 30-110xc2x0 C., the reaction pressure is 0.1-5.0 MPa, and the gas flow rate in the reactor is 10-100 cm/sec., but these factors can be appropriately selected by specialists in the art.